Combustion control system

ABSTRACT

A COMBUSTION CONTROL SYSTEM INCLUDES A FUEL CONTROL, A FLAME RELAY, AN IGNITER AND A FLAME SENSOR FOR SENSING THE PRESENCE OF FLAME IN THE COMBUSTION CHAMBER. THE CONTROL CIRCUITRY IS ARRANGED TO PERMIT FLOW OF FUEL INTO THE COMBUSTION CHAMBER IN RESPONSE TO A REQUEST FOR IGNITION AND A RESISTIVE CAPACITIVE TIMING CIRCUIT PERMITS THE FLOW OF FUEL INTO THE COMBUSTION CHAMBER FOR A PREDETERMINED TIME AND THEN TERMINATES THE FLOW OF UEL UNLESS THE FLAME SENSOR INDICATES THAT FLAME IS PRESNT BEFORE THE END OF THE PREDETERMINED TIME INTERVAL.

May 23, 1972 P. J. CADE COMBUSTION CONTROL SYSTEM Filed Aug. :5, 1970mm/ mm \QQN kQ m Q wmw w wnwnew W? N A o 8 /W kww mm mmkw amn in T k QQmm mm .H. 3 kb@ United States Patent 3,664,803 COlVIBUSTION CONTROLSYSTEM Phillip J. Cade, Winchester, Mass., assignor to ElectronicsCorporation of America, Cambridge, Mass. Filed Aug. 3, 1970, Ser. No.60,230 Int. Cl. F23n 5/20 US. Cl. 431-69 8 Claims ABSTRACT OF THEDISCLOSURE A combustion control system includes a fuel control, a flamerelay, an igniter and a flame sensor for sensing the presence of flamein the combustion chamber. The control circuitry is arranged to permitflow of fuel into the combustion chamber in response to a request forignition and a resistive capacitive timing circuit permits the flow offuel into the combustion chamber for a predetermined time and thenterminates the flow of fuel unless the flame sensor indicates that flameis present before the end of the predetermined time interval.

This invention relates to combustion control systems.

In combustion control systems, the existence of a flame in thecombustion chamber is conventionally supervised by a sensor such as aflame rod or an infrared or an ultraviolet radiation detector. Prior toignition, the sensor generates a signal indicating that no flame ispresent, after flame is established in the combustion chamber the sensorgenerates a signal indicating that flame is present and upon flame fail,a flame failure signal is generated. In typical ignition sequence, fuelis allowed to flow into the combustion chamber for ignition prior toestablishment of flame. If flame is not established promptly, the fuelflow is terminated and a purge sequence is provided prior to restart ofthe ignition sequence to clear the unburned fuel from the combustionchamber. The trial for ignition" interval must be terminated after apredetermined time or unburned fuel will continue to flow into thecombustion chamber, creating a hazardous condition. A variety of circuitarrangements for timing this trial for ignition interval have beensuggested. In a conventional arrangement, a motor driven timer isemployed which closes a switch to prevent the flow of fuel into thecombustion chamber for a predetermined interval and terminates that flowof fuel if flame ignition is not established within that interval. Sucha system can fail unsafely in that the trial for ignition period couldbe indefinitely long if the motor driving the cam should stall.Sometimes a thermal lockout switch is used to avoid this problem. Thelockout switch heater is energized during the trial for ignition periodso that the lockout switch operates if the timer motor stalls. Suchlockout switches are not sufliciently accurate for short trialintervals, such as two to fifteen seconds, and also the timing isdependent on voltage.

It is an object of this invention to provide a novel and improvedcombustion control system.

A more specific object of the invention is to provide novel and improvedcircuit arrangements for limiting the trial for ignition interval.

Another object of the invention is to provide novel and improvedcombustion control circuit arrangements that provide a reliable trialfor ignition interval in a simple and inexpensive manner.

Another object of the invention is to improve safe burner operation byproviding improved means for limiting the interval during which fuelflows to a burner without being ignited. Further objects are to effectthe limitation simply, inexpensively, and in a fail-safe manner.

In accordance with the invention there is provided a combustion controlsystem having a main fuel valve for 3,664,803 Patented May 23, 1972controlling the flow of fuel to a combustion chamber, a relay forcontrolling the main fuel valve, an igniter in the combustion chamberfor igniting fuel in the combustion chamber to produce flame, a flamesensor for sensing the presence of flame in the combustion chamber, andcontrol circuitry for opening the main fuel valve to permit flow of fuelinto the combustion chamber in response to a request for ignition, and atiming circuit consisting solely of passive components for producing anoutput signal permitting flow of fuel into said combustion chamber for apredetermined time interval and terminating the flow of fuel to thecombustion chamber in the absence of a flame present signal from theflame sensor before the end of the predetermined interval.

In a preferred embodiment the circuitry includes a capacitor and aswitch responsive to a trial for ignition request. The capacitor isconnected to a source of power prior to initiation of the ignitionrequest and in response to the ignition request is disconnected from thesource of power and connected to circuitry to operate the fuel control.In a particular embodiment, this switch is a cam operated single poledouble throw switch that, in a first position, connects the capacitor toa source of power and in a second position connects the capacitor in adischarging circuit to control the operation of the fuel control. Inthis embodiment the circuit between the capacitor and the fuel controlincludes a flame relay that is connected to respond to the flame presentsignal from the flame sensor and the capacitor is connected in a timingcircuit with at least one resistor so that the trial for ignitioninterval is determined by the time constant of the capacitor and theresistor and energizes the flame relay during the trial for ignitioninterval independent of the signal from the flame sensor.

Other objects, features, and advantages will appear from the followingdescription of a preferred embodiment of the invention taken togetherwith the attached drawing which shows a diagram of a burner controlconstructed in accordance with the invention.

Referring to the figure, terminals 12 and 14 provide connections for asuitable source of AC power supplied to transformer 16. Flame detector 18 employing ultraviolet sensitive tube 20, is connected in circuit withfirst secondary winding 22 of transformer 16 and primary winding 24 ofreadout transformer 26. Secondary winding 28 of transformer 26 isconnected with diodes 30, 32 to form a full wave rectifier circuit, theoutput of which is connected through lead 34 to the base electrode oftransistor 36. A second secondary winding 38 of transfonner 16 isconnected in a circuit with diode 40 and capacitor 42 to provide a B+power supply. Electronic circuitry connected to this power supplyincludes transistors 36 and 46 and controls the flow of current throughrelay coil 48. Relay 48 governs the valve 50 that controls the admissionof fuel to combustion chamber 52. An igniter 54 is disposed in chamber52. A motor drives cams which operate contacts 60-1, 60-2 and 60-3.Contacts 60-1 and 60-2 are arranged as a single pole, double throwswitch 60 that has a first position in which capacitor 62 is connectedvia contacts 60-1 through resistor 64 to the B-I- supply and a secondposition in which capacitor 62 is connected via contacts 60-2 to avoltage divider consisting of resistors 66 and 68, the center junction70 of which is connected to the base of transistor 72 and its collectorelectrode is connected to the base electrode of transistor 46.Additional details of a combustion control system incorporatingapparatus of this type may be had with reference to co-pendingapplication Ser. No. 60,229, entitled Electrical Control Apparatus,filed concurrently herewith and assigned to the same assignee as thisappli cation.

The operation of the apparatus is as follows:

In the pre-ignition period valve 50 is closed, contact 60-1 is closed,and contacts 60-2, and 60-3 are open. Flame sensor 20 sees no flameradiation and is therefore in a nonconductive state. No current flowsthrough winding 24 of transformer 26 and there is therefore no output onlead 34. Transistor 36 is therefore in ofl? (non-conductive) condition.Transistor 72 is also in non-conductive state with its base electrode atapproximately ground voltage. Approximately B-lvoltage is thereforeapplied to the base of electrode transistor 46 with the result thattransistor 46 is also non-conductive and no current flows through relaycoil 48. The fuel valve 50 controlled by relay 48 is thus closed.Capacitor 62 is maintained during this period in a charged condition byconnection through switch contact 60-1 to the B+ power supply.

A trial for ignition mode of operation is initiated when cam operatedswitch 60 is switched thereby opening contact 60-1 and closing contacts60-2 and 60-3. Capacitor 62 thereupon supplies a voltage through contact60-2 and resistor 66 to the base of transistor 72, causing transistor 72to become conductive and lowering the voltage on the base of transistor46, which in turn becomes conductive allowing current to flow throughrelay coil 48. The fuel valve 50 is thereupon opened. If the fuelflowing into chamber 52 is ignited, ultraviolet sensor tube 20 inresponse to the flame becomes conductive, and current flows throughwinding 24 of transformer 26 with the result that a positive voltage isapplied through lead 34 to the base of transistor 36. Transistor 36thereupon becomes conductive and maintains a reduced voltage on the baseof transistor 46 so that transistor 46 remains conductive, and currentflow is maintained through relay coil 48. The fuel valve 50 thuscontinues in open condition. However, capacitor 62 is being dischargedthrough resistors 64, '66 and '68. The time constant for this dischargeis the product of the capacitance of capacitor 62 times the sum of theresistances of resistors 64, 66 and 68. The voltage on the base oftransistor 72 will decline and that transistor will go to non-conductivestate. If ignition in chamber 52 is not achieved within a time intervalthat is a function of that time constant, the voltage on the base oftransistor 46 will thereupon rise so that transistor 46 will becomenonconductive with the result that the current through relay 48 andconsquently fuel valve 50* is closed. The apparatus thus simply,reliably and in fail safe manner limits the interval during which fuelflows without ignition to approximately the time constant of capacitor62 with resistors 64, 66 and 68 in series.

While a particular embodiment of the invention has been shown anddescribed, various modifications thereof will be obvious to thoseskilled in the art. For example, it will be obvious that principles ofthe invention are applicable to circuitry using other types of flamedetectors. Also, other forms of switches and charging circuitry may beemployed in the practice of various aspects of the invention. Therefore,it is not intended that the invention be limited to the disclosedembodiment or to details thereof, but departures may be made therefromwithin the spirit and scope of the invention.

What is claimed is:

1. A combustion control system comprising a valve for controlling theflow of fuel to a combustion chamber, a flame sensor for sensing thepresence of flame in said combustion chamber and producing a flamepresent signal in response thereto, a flame relay connected to respondto said flame present signal from said flame sensor for controlling saidfuel valve, an igniter in said combustion chamher for igniting fuel insaid combustion chamber to produce flame, control circuitry responsiveto said flame sensor for energizing said relay to operate said fuelcontrol and permit flow of fuel into said combustion chamber, and atiming circuit consisting solely of passive components for producing anoutput signal in response to a request for ignition that defines a trialfor ignition interval, said output signal energizing said flame relayduring said trial for ignition interval independent of the signal fromsaid flame sensor and permitting flow of fuel into said combustionchamber for a predetermined time interval and terminating the flow offuel to the combination chamber in the absence of a flame present signalfrom said flame sensor before the end of said predetermined interval.

2. The system as claimed in claim 1 wherein said timing circuit includesa capacitor and a switch responsive to a trial for ignition request,said capacitor being connected to a source of power prior to initiationof the ignition request and said switch, in response to the ignitionrequest, disconnecting said capacitor from the source of power andconnecting said capacitor to energize said flame relay.

3. The system as claimed in claim 2 wherein said switch is a camoperated single pole double throw switch which, in a first position,connects said capacitor to said source of power and, in a secondposition, connects said capacitor in a discharging circuit to controlthe operation of said fuel control.

4. The system as claimed in claim 1 wherein said timing circuit includesa capacitor and further including a circuit between said capacitor andsaid flame relay said capacitor being connected in said timing circuitso that the trial for ignition interval is determined by the timeconstant of said timing circuit and said flame relay is energized duringsaid trial for ignition interval independent of the signal from saidflame sensor.

5. In control apparatus for a burner including a flame detector, a relayadapted to effect withholding of fuel to the burner when said relay isin its normal state and to effect admission of fuel when in itsenergized state, circuitry adapted to receive a signal from said flamedetector when a flame is detected and thereupon to effect the energizingof said relay, and a switch adapted to provide a trial-for-ignition modeof operation of said apparatus,

a capacitor adapted to be connected to a source of power prior toinitiation of said trial-for-ignition mode and to be disconnected fromsaid source during said mode, and means to connect said capacitor incircuit with said relay to provide for a limited interval subsequent tosaid initiation a voltage effecting the energizing of said relay.

6. The apparatus of claim 5 in which a single pole, double throw switchin a first position connects said capacitor to said source and in asecond position connects said capacitor to second circuitry controllingthe energizing of said relay.

7. The apparatus of claim 4 in which said single pole, double throwswitch is actuated by a timing cam.

8. The apparatus of claim 5 in which said apparatus includes an igniterfor igniting said fuel and further including means for energizing saidigniter concurrently with the connection of said capacitor in circuitwith said relay.

References Cited UNITED STATES PATENTS 3,270,800 9/ 1966 Deziel et al431--69 3,488,131 1/1970 Myers et a1 43169 X 3,276,507 10/1966 Eldridge,Jr. et a1 431-69 3,350,581 10/1967 Stein, Jr 307-117 3,388,566 6/1968Kaper 307-117 X EDWARD G. FAVORS, Primary Examiner

